Over the last month I've been tracking down some rain water leaks in my 1983 G30. First it appeared that it was a leaking fuse box to firewall seal so I replaced that and got it all tight, then I discovered it was a more major leak from a rusted out seam under the hood cowl right in the center. This was leaking behind the doghouse cover, would saturate the insulating foam and then drip down into the passenger and drivers feet areas. That was easy to seal up from the cowl side. Unfortunately I found that I was also getting water in from the heater box to firewall seal.

I took the dash entire off and removed the heater box and don't have any issue getting a good seal when I replace it, however I'm thinking i'm still going to have issues with water getting into the A/C evaporator housing on the other side of the firewall. I've been referencing this post: https://www.vanning.com/threads/ubbthreads.php/topics/308091/some-of-my-latest-headaches as G_10 had a lot of good photos of his evap/heater box rebuild, but he had the benefit of a disconnected A/C system which leads me to my question.

Is it possible to separate the evaporator housing from the firewall enough to reseal it all the way around with butyl tape without disconnecting the pressure hoses and evacuating the R12 from the system?

My guess is no, but I thought I'd ask in case someone has some great wisdom.